According to PMBUS v1.3.1 part II specification the used prefix 0xff for extended
commands is reserved for future use:
"The PMBUS_COMMAND_EXT is reserved for future use to extend the PMBus command set
beyond the available 256 command codes."

As an alternative the standard points the MFR_SPECIFIC_COMMAND_EXT to be available
with 0xFE as the first byte of a command.
The same standard also points that standard commands from 0xc4 can be defined
by a manufacturer. To be compliant with the standard and to simplify the design
I have decided to move all used extended command to the standard range.
Be aware that this brings some non-backward compatible changes.

Changes include:
- main FW
- bootloader
- test_master
- documentation
Signed-off-by: Adam Wujek <dev_public@wujek.eu>